It is rather a speculation drawn from Lovecraft's work that the old gods were based on. In Lovecraft mythos there is a being named the Azathoth an eldritch deity outside the universe whose the ancestor to other elder gods.

In this family tree, Azathoth is positioned as a primordial being, and the sole parent of Nyarlathotep, the Nameless Mist and Darkness. Through these beings, Azathoth is the direct ancestor of Yog-Sothoth, Shub-Niggurath, Cthulhu, Tsathoggua and many other deities.

C'thun is based on Cthulhu,
Yogg-saron is based Shub-Niggurath,
Y'shaarj is based on Nyarlathotep,
N'zoth is based on Yog-Sothoth

So within the context of WoW, it is speculated that the planet Azeroth is a titan prison built around the entity Aze'roth whose manifestations are the Old Gods and over the years the Old Gods have seeped-deep into the crust of Azeroth almost fusing with it. Hence, their destruction would mean the destruction of the crust of the planet which would lead into the releasing of Aze'roth etc.

Which of his books do I need to read to get a feel for these old Gods?

Wonder what the odds are on 'The Dark Below' actually being the next expansion for SC2 - since you'd expect that would probably come out around the same time as the D3 one (half way between 5.4 and the next WoW expansion).

On the new Battle.Net launcher Beta application,
when you click on the WoW or StarCraft II tab it will show up in grey text "Upgrade Available".
But when you click on the Diablo III tab the text does not appear.

I have all my games up to the latest expansions,
maybe "The Dark Below" it related to either WoW or SC II?

Wonder what the odds are on 'The Dark Below' actually being the next expansion for SC2 - since you'd expect that would probably come out around the same time as the D3 one (half way between 5.4 and the next WoW expansion).

Which of his books do I need to read to get a feel for these old Gods?

The Old Gods aren't straight expys of the Great Old Ones, but they're definitely inspired by Lovecraft's Mythos, within which Shub-Niggurath, Nyarlathotep, and Yog-Sothoth are technically Outer Gods rather than Great Old Ones like Cthulthu. Most of Lovecraft's work was done in the short story form. Probably the two best collections are Necronomicon: The Best Weird Tales of H.P. Lovecraft and Eldritch Tales: A Miscellany of the Macabre. For short stories I'd recommend starting with At the Mountains of Madness, A Shadow Out of Time, and The Dunwich Horror.